Lignocellulosic biomass represents an abundant available raw material for the production of bio-fuels and bio-chemicals such as ethanol, butanol, lactic acid and succininc acid. It is composed of carbohydrate polymers (cellulose and hemicelluloses), and an aromatic polymer (lignin). The carbohydrate polymers contain different sugar monomers (six and five carbon sugars) and are tightly bound to lignin.
Lignocellulosic biomass is a potential source of sugars for the production of fuels and chemicals as it is relatively cheap and abundant. However, a significant challenge to the cost-competitive generation of sugars from biomass is the low accessibility of its polysaccharide components to hydrolytic enzymes and chemicals. This phenomenon is known as biomass recalcitrance and is largely governed by the presence and physicochemical properties of lignin in the plant cell wall. In addition to physically preventing the accessibility of cellulose and hemicelluloses to cell wall degrading enzymes (cellulases and hemicellulases), lignin contributes to biomass recalcitrance through hydrophobic interactions with the enzymes, decreasing their effective concentration. As a result, a pretreatment is required to reduce biomass recalcitrance.
However, because of the lack of selectivity of leading pretreatment technologies towards lignin, this type of fractionation/cell wall degradation is challenging. For example, while mild pretreatment conditions result in high carbohydrate recovery, the pretreated biomass retains the majority of its recalcitrance. On the other hand, more severe conditions typically generate substrates that are more amenable towards enzymatic saccharification. Unfortunately, this increase in susceptibility to enzymatic hydrolysis is accompanied by severe carbohydrate losses and the accumulation of carbohydrate-degradation products such as furfural and levulinic acid, which are inhibitory to downstream processes. As a result, pretreated biomass usually contains a substantial amount of residual lignin that remains associated with the carbohydrates (Chandra et al., 2007, Adv Biocehm Engin/Biotechnol, 108: 67-93).
Unfortunately, the residual lignin has a substantial effect on the enzymatic hydrolysis of pretreated lignocellulosic biomass. More specifically, lignin acts as both a physical barrier, which restricts the accessibility of carbohydrate-degrading enzymes to their substrates (Mooney et al., 1998, Biores. Technlo., 64: 113-119), and as an “attractant” to cellulases and hemicellulases, which decreases their effective concentration through non-productive binding (Yang and Wyman, 2006, Biotech. Bioeng., 94: 611-617). Consequently, relatively high enzyme loadings are required to achieve high carbohydrate conversion yields in a short time. Therefore, the need to decrease the enzyme loadings needed to achieve high carbohydrate 80%) conversion yields within a reasonable time period (≦72 h) is a challenge that needs to be addressed to improve the process economics.
There is thus still a need to be provided with a process for improving the conversion of lignocellulosic biomass to commercially useful products.